1. Field of the Invention
The present invention relates to a method of adjusting the stress of an X-ray mask.
2. Description of the Background Art
In the conventional pattern transfer process of a semiconductor memory device that does not have such a high integration density, a lithographic technique employing ultraviolet light was mainly used. However, as semiconductor memory devices are scaled to higher densities such as a 1G-bit (gigabit) DRAM (Dynamic Random Access Memory), pattern transfer of higher resolution is required since each pattern such as the interconnection pattern is in the submicron range according to the device rule.
Attention is directed to lithography employing X-rays for such microscopic patterns. In the X-ray lithography technique, the wavelength of the X-rays (soft X-rays: .lambda.=5-20 nm) emitted as the exposure light is shorter than that of ultraviolet light. Therefore, a pattern having a resolution higher than that produced by ultraviolet light lithography can be transferred.
Fabrication of an X-ray mask employed in such X-ray lithography is disclosed in Japanese Patent Laying-Open No. 7-135157 by the applicant of the present application.
A method of fabricating an X-ray mask disclosed in the publication thereof will be described hereinafter as conventional art.
FIGS. 38-44 are sectional views of such a conventional X-ray mask indicating the sequence of a fabrication method thereof.
Referring to FIG. 38, a membrane (a synonym for an X-ray transmittance substrate) 2 is formed on a silicon substrate 1.
Referring to FIG. 39, a portion of silicon substrate 1 is selectively removed (back etching) until the back surface of membrane 2 is partially exposed.
Referring to FIG. 40, an anti-reflective film also serving as an etching stopper film (referred simply as "anti-reflective film" hereinafter) formed of, for example, indium-tin oxide (ITO) is applied or sputtered on membrane 2. Then, a baking process is applied.
Referring to FIG. 41, an X-ray absorber 4 of a material that blocks transmission of X-ray is formed by, for example, sputtering, on anti-reflective film 3. Here, the average film stress of X-ray absorber 4 is measured. Annealing is carried out uniformly in an oven, for example, at 250.degree. C. to attain an average film stress of 0.
Referring to FIG. 42, following application of a resist 5 on X-ray absorber 4, a baking process is carried out at 180.degree. C., for example.
Referring to FIG. 43, a support ring 6 is attached to the bottom surface of silicon substrate 1 by an adhesive 7. Resist 5 is patterned by electron beam (EB) exposure and development. Using the patterned resist 5 as a mask, X-ray absorber 4 is subjected to dry etching, whereby X-ray absorber 4 is patterned. Then, resist 5 is removed. Thus, the X-ray mask shown in FIG. 44 is produced.
In the conventional stress adjustment method of an X-ray mask, the average film stress of X-ray absorber 4 is set to 0 right after formation of X-ray absorber 4 during the fabrication process of an X-ray mask.
Although X-ray lithography is applied in the transfer process of microscopic patterns due to the short wavelength of the X-ray, this transfer is generally in equal scale due to the nature of an X-ray. Therefore, high position accuracy of the pattern is required for the X-ray mask. When there is residual stress in X-ray absorber 4 prior to the patterning step of X-ray absorber 4, the pattern position of X-ray absorber 4 will be shifted by that stress after the patterning process of X-ray absorber 4 to result in degradation of the position accuracy.
In the conventional method of fabricating an X-ray mask, annealing is carried out before the patterning process of X-ray absorber 4 so that the average film stress of X-ray absorber 4 is adjusted to be 0.
However, the inventors of the present invention found that the pattern position of X-ray absorber 4 is shifted after the patterning process even if the average film stress of X-ray absorber 4 is set to 0 prior to the patterning process. The inventors extended intensive efforts to elucidate the cause. They have come to the following five factors which will be described in detail hereinafter.
(1) As shown in FIG. 40, the baking process is carried out after application of anti-reflective film 3. The surface of anti-reflective film 3 is oxidized as shown in FIG. 45 by this baking, whereby an oxide film 3a is formed. Compressive stress is generated in oxide film 3a by introduction of oxygen. PA1 (2) Following the patterning of X-ray absorber 4 of FIG. 44, an oxide film 4a is formed by native oxidation as shown in FIG. 47 at the sidewall of X-ray absorber 4. At this oxide film 4a, compressive stress is generated by introduction of oxygen. The formation of oxide film 4a at a sidewall causes compressive stress in the direction indicated by the arrow to be exerted at the side of X-ray absorber 4. As a result, the pattern position is shifted. PA1 (3) In the patterning step of X-ray absorber 4 shown in FIGS. 43 and 44, the sidewall of X-ray absorber 4 may be patterned obliquely as shown in FIG. 48(a) to exhibit the phase shifting effect. In this case, the phase of the transmitted light of sidewall 32 differs relative to the phase of the transmitted light of transmitting portion 31 as shown by the dotted line in FIG. 48(b). In the overlapping area of the exposure light transmitted through transmitting portion 31 and the exposure light transmitting through sidewall portion 32, the exposure light may cancel each other. As a result, a region is generated where the light intensity is reduced as shown by the solid line to facilitate transfer of a microscopic pattern. The solid line in FIG. 48(b) indicates the sum of the light intensity of light transmitting through each portion. PA1 (4) The X-ray mask is used in X-ray lithography. In the case where anti-reflective film 3 of FIG. 44 is formed of, for example, SiO.sub.2, the bond between Si and O of SiO.sub.2 is broken by projection of the X-ray to alter the value of stress. As a result, the pattern of X-ray absorber 4 is offset in position. PA1 (5) In the case where there is a defect in the patterned X-ray absorber 4, the defect must be corrected by means of FIB, laser, and the like. However, defect correction of a great range will cause position offset in the X-ray absorber pattern.
Following adjustment of the average film stress of X-ray absorber 4 to 0, etching is carried out to pattern X-ray absorber 4 as shown in FIGS. 43 and 44. When oxide film 3a located at the top surface of anti-reflective film 3 is selectively removed by this etching as shown in FIG. 46, the compressive stress is eliminated at the removed portion. The stress of this portion becomes tensile stress. As a result, the pattern of X-ray absorber 4 is pulled in the direction indicated by the arrow. Thus, offset is generated in the pattern position.
The general stress distribution within X-ray absorber 4 is shown in FIG. 49. Compressive stress is exhibited at the lower side of X-ray absorber 4, and tensile stress is exerted at the upper side of X-ray absorber 4. When a pattern of X-ray absorber 4 having an oblique sidewall as shown in FIG. 50 is formed, the compressive stress and the tensile stress are offset to result in an average film stress of 0 at the portion 4b.sub.2 having a cross section of substantially a quadrilateral. However, the compressive stress will become greater at the portion 4b.sub.1 where the cross section is substantially a triangle.
As a result, the sidewall of the X-ray absorber pattern exhibits compressive stress as shown in FIG. 51 to cause pattern position offset in the direction indicated by the arrow.